The present invention relates to a multi-tone transceiver system for the transmission and reception of high-speed digital data at a rate of tens of Mbps through phone lines, and more particularly, to a multi-tone transceiver system having a widened bandwidth that can be interoperable with Asymmetric Digital Subscriber Line (ADSL) transceiver system.
Most popular techniques for a digital subscriber line transmitting high-speed digital data are Discrete Multi-Tone-based Asymmetric Digital Subscriber Line (DMT-based ADSL) which is commercialized recently, and Very-high-speed Digital Subscriber Line (VDSL), which is proceeding for a standard.
The DMT transceiver system is a digital transmission technique for transmission and reception of multi-carrier, and the DMT-based ADSL is a system for transmitting high-speed digital data over the common telephone line by locating IFFT on transmission side and FFT on receiving side.
The DMT transceiver system uses 256 tones that are each 4.3125kHz wide in the downstream direction. FIG. 1 is a block diagram illustrating system architecture.
In the described DMT transceiver System, an input signal of modulator is Quadrature Amplifier Modulation (QAM) symbol type signal completed in channel coding in early phase. Considering one block as an input from 1 to 512, 1st and 257th tone signal is zero, 2xcx9c256 tone signals are delivered from early phase, and the others are arranged in the type of signals, in Hermitian symmetric relation with 2xcx9c256th tone signals. This arranged block of signal input to the side of Inverse Fast Fourier Transform (IFFT), and the output of the side of IFFT becomes real number signal by exponential modulation. These processes are the method of DMT modulation at transmission side.
Next, the modulated symbol block of real signal is converted to serial signal by Parallel to Serial (P/S) transformer 12, than transmitted over a channel 13 in the type of serial signal. Within this process, cyclic prefix is attached prior to the block in the purpose of removing the effect of transmission channel. For example, in the case of DMT system using the bandwidth of 1.104MHz, one symbol per each tone utilizes the bandwidth of 4kHz and 0.3125kHz exists for the protection bandwidth. As a result, the number of cyclic prefix is 512xc3x97(0.3125kHz/4kHz)=40.
In the described DMT transceiver system, a receiving signal on the side of DMT demodulator contains noise in the process of signal transmission over channel from transmitter by a channel attenuation effect. After striping cyclic prefix from the received signal, the S/P transformer unit 15 transforms it to parallel signal. To facilitate this, this real vector of parallel signal transformed to the type of QAM symbol by the FFT unit 16. This is the method of the DMT demodulation.
The above-described DMT transceiver system divide transmission bandwidth to each tone using the orthogonal characteristic of IFFT/FFT, that is, the transmission bandwidth is divided into plural tones by utilizing filters that is orthogonal each other, and the corresponding filters for each tone performs an exponential modulation. But one acknowledged drawback of this system is that the cross-talk noise is significant between tones, since a difference between main node and sub node for filters, which separate the transmission bandwidth, is just 13 dB. For ideal channel, that is, a response length of impulse is shorter than that of one symbol, the signal transmitted from transmitter can be restored on receiving side completely without interference between tones since symbols of each tone have no effect on others. However, in practical channel environment of transmission, the response length of impulse is relatively long so that symbols of each tone have affected others. This effect is significant in the case of DMT system, the difference between main node and sub node is just 13dB, which divides each tone. In order to eliminate the undesirable effects between symbols, there must be enough protection area to eliminate them on the channel. In general, this protection area is called cyclic prefix, and it should be bigger than xe2x80x9cthe response length of channel impulsexe2x88x921xe2x80x9d in order to eliminate the interference between symbols. Nevertheless, the cyclic prefix is not a concerned data, the longer its length, the less real data capacity. Additional drawback of DMT system is that it is easily affected by burst noise. That is, even the burst noise has characteristics of narrow bandwidth, the difference between main node and sub node of the transceiver filter, which consists of DMT system, is just 13dB so that it has affected not only tones but also it has affected adjacent tone significantly.
As a solution for that, Time-domain Equalizer (TEQ) 14 is suggested to use illustrated in FIG. 1. TEQ is used to reduce the response length of the channel impulse, which causes Inter-Symbol Interference (ISI) so that it reduces the length of cyclic prefix ultimately. In case of general transmission channel, the length of impulse including major energy is not significantly long. Therefore if it can be collected within the certain objective length, the response length of channel impulse affected in practice can be reduced. Accordingly, using the adjustment equalizer such as TEA in the process of initializing modem can reduce the response length of the channel impulse affected in practice, so that the length of cyclic prefix can be reduced.
In the preceding description of DMT transceiver system, Frequency-domain Equalizer (FDE) 17 is used to compensate the attenuation and phase transformation caused by channels.
On the other hand, VDSL has submitted for standardization process, which is the technique for transmitting high-speed data over public telephone line faster than ADSL. Comparing to ADSL, VDSL uses broader bandwidth and transmits with better quality of data over the telephone line in relatively short distance.
As well known transmission methods of VDSL are QAM (Quadrature Amplitude Modulation) based SCM (Single Carrier Modulation) and FFT based DMT, however none of those are standardized in VDSL.
FIG. 2 is a block diagram illustrating general concept of DMT-based VDSL (Discrete Multi-Tone-based Very-high-speed Digital Subscriber Line) At the DMT-based VDSL transceiver system, a transmitter includes IFFT unit 21 and P/S transformer unit 22, and a receiver includes TEQ 24, P/S transformer unit 22, FFT unit 26 and FEQ 27. The operation of each unit in system is similar or same in accordance with FIG. 1.
Above-described DMT-based VDSL transceiver system is designed so that the bandwidth of each tone is set to 4.3125 kHz similar with ADSL transceiver system, and its utilizing bandwidth is integral number times of bandwidth of tone. In general, the standardization group set the maximum utilizing bandwidth to 17.664MHz that is 16 times of maximum utilizing bandwidth of ADSL transceiver system considering the interoperability with it. Accordingly, the maximum number of tones used in DMT-based VDSL transceiver system is 4096, and modulation/demodulation should be performed using by 8192-point IFFT/FFT.
It should be noted that, what DMT-based VDSL transceiver system is different to DMT-based ADSL transceiver system is that the size of IFFT/FFT should be bigger due to its wider bandwidth. Nevertheless, considering the present technology related to semiconductor and complexity, there is a size limit to implement FFT, consequently the allocated bandwidth cannot be utilized in complete. Another difference is that the number of TEQ tabs and cyclic prefix are big since the bandwidth used in DMT-based VDSL is wide. In addition that the operation speed of TEQ should be also faster, since the more the bandwidth usage, the more the sampling speed. That is, in case of DMT-based ADSL transceiver system using the bandwidth 1.104MHz, it utilizes about 30 tabs of TEQ and its operation speed is 2.208MHz. On the contrary, in case of DMT-based VDSL transceiver system using the bandwidth 17.664MHz, which is 16 times of 1.104MHz, it utilizes more than 400 tabs of TEQ and its operation speed is 35.328MHz. This is a significant overhead and possibly causes the delay to initialize a modem.
For multi-carrier transmission, a filter is required which can divide transmission bandwidth in to plural tones. There are two kinds of method to design a filter for this purpose. First method is to design a filter, which has a comparably wide passband and a very narrow transition band. But, to satisfy the above conditions, a very high order filter is required. Second method is to design a filter, which maintains the good orthogonal characteristic though the bands of the filter are overlapped. This method can effectively reduce the filter""s order but design condition is very complicated. One of the multi-carrier transmission method, using the second filter design method mentioned above, is the one which utilizes a wavelet, and is called Discrete Wavelet Multi-tone (DWMT) transmission method. On the contrary that DMT system generally uses FFT for modulation and demodulation, DWMT uses wavelet instead of FFT. FIG. 3 illustrates the above as a simple block diagram. Wavelet block can be constructed by using Cosine-Modulated Filter Bank (CMFB) due to the fact that CMFB is beneficent in simplicity of filter design compared to other wavelet systems and in a system structure which can use fast algorithm. And, the filters, comprising the CMFB, are to be designed to have the orthogonal characteristic. But, CMFB is designed through a nonlinear optimization process so, if the size is big, the design process becomes very complex hence, it is impossible to design up to some extent of size, which is its drawback.
CMFB is a system comprised of the demultiplexer bank and the multiplexer bank. The demultiplexer bank divides an input signal in to plural frequency components and the multiplexer bank combines the divided frequency components in to one frequency component to recover the orginal signal""s frequency response. Accordingly, the most important fact in designing the CMFB is that this system should be complete recovering system. CMFB design method is on that the original filter, which satisfies complete recover conditions, should be made first and then the multiplexer and the demultiplexer should be made by gathering the cosine modulated filters of these original filters. At this time, the bands, occupied by the each modulation filter, are called sub channel.
As illustrated in FIG. 4, by placing the multiplexer bank at modulation part and by placing the demultiplexer bank at demodulation part, it can be used as a transmuliplexer. The difference between this DWMT system and FFT based DMT is in that the original filter design and the characteristics of the processed signals. On the contrary that FFT has the fixed original filter by a rectangular window, CMFB makes it possible to design original filter as a user wanted type. Accordingly, by designing an original filter, which has a big difference between main and sub node, interference between sub channels can be minimized. On the contrary that FFT is exponential modulation and correspondingly the processed signals are complex numbers, but CMFB is the cosine modulation and processed signal is real numbers. DWMT transceiver is a system, which minimizes the interference between the modulation filters by applying said CMFB to modulation and demodulation, it removes the necessity of cyclic prefix, which is the fundamental drawback of DMT, and also minimizes the effect of burst noise. However, it has drawbacks such as a widened bandwidth, a technical difficulty of realizing the modulation filter in accordance with the increased tones, and a complexity of filter construction to compensate the frequency attenuation due to the channel.
The summary of the above detailed description is as follows.
The complexity of the DMT based VDSL transceiver system, presented in general conception, increases as the bandwidth increases. In general, if the size of FFT is L (L-point FFT), the complexity is expressed as Llog2L, so when L increases 16 times, FFT complexity is supposed to increases 64 times. The problem is on the limitation of realization aspect of FFT, accordingly there is a limit on the size of L. Presently, it is very burdensome to realize and use more than 2048-point FFT. The reason for that is the size becomes big and the processing speed becomes slow. Accordingly, it is inevitable to give a limitation on bandwidth in designing a wideband DMT transceiver system, which is interoperable with ADSL transceiver system. The other problem is on that the length of TEQ should be linearly increased as the bandwidth is increasing, and this huge length filter should operate in high-speed. Some engineers are to construct a system without using TEQ due to the said reasons. If a system is constructed without using TEQ, performance of the system can be largely varied in accordance with the cyclic prefix length determination. But, the length of cyclic prefix can not be varied every time whenever the channel is changing so, if one intends to remove channel""s effect only with the cyclic prefix, system""s performance is drastically dropped if the cyclic prefix is not long enough.
The presented invention is devised to solve the problems of previous techniques. And the invention is to provide the wideband multi-tone transceiver system, which is effective in the hardware aspect and interoperable with Asymmetric Digital Subscriber Line (ADSL) transceiver system.
To achieve the above goals, the multi-tone transceiver transmitter, constructed with the presented invention of two-steps DMT-CMFB, includes DMT modulator which divides the channel coded input signal""s bandwidth in to plural sub bands and performs multi-carrier modulation of those sub bands and outputs the real part of the modulated signal. And it also includes multiplexer bank of CMFB transmultiplexer which transmits real part of the signal to the channel by frequency division multiplexing. The receiver includes the demultiplexer bank of the CMFB transmultiplexer, which divides the said frequency multiplexed signal, sent through the channel, into each frequencies and separates them as each sub band signals. And also includes the DMT demodulator which performs multi-carrier demodulation of the real part of said separated sub bands and recovers an original signal which is before the modulation in transmitter.
Preferably, at least the lowest sub band among the above separated sub bands should include the ADSL band.
Additionally, the transmitter of multi-tone transceiver system, consisted of the presented invention of two-steps DMT-CMFB, includes, the first buffer which temporary stores channel coded signal, input from the front-end, and sequentially outputs as plural separated sub bands, the DMT modulator which performs multi-carrier modulation of the sub band signals. And, also includes the second buffer which temporary stores the real part of the multi-carrier modulated sub bands, and the multiplexer bank of the CMFB transmultiplexer which performs the frequency division multiplexing of the output signals from the said second buffer and sends to the channel. The receiver includes demultiplexer bank which divides the frequency division multiplexed signals, sent through said channel, in to each frequencies and separates to each sub band signals, and the third buffer which temporary stores the real part of said separated sub band signals. And it also includes the DMT demodulator, which performs multi-carrier demodulation of the sub and signals, output from the said third buffer, and the fourth buffer which temporary stores the said demodulated sub band signals and outputs the original signal of the whole bandwidth.
Preferably, at least the lowest sub band among the above separated sub bands should include the ADSL band.
More preferably, the above second buffer is constructed by the double buffer structure which includes a buffer that temporary stores the output signal from the DMT modulator, and a buffer that temporary stores the signal which is to be transferred to the multiplexer bank of said CMFB transmultiplexer.
Further preferably, the above third buffer is constructed by the double buffer structure which includes a buffer that temporary stores the output signal from the demultiplexer bank of CMFB transmultiplexer, and a buffer that temporary stores the signal which is to be transferred to the DMT demodulator.
Further preferably, it includes the time domain equalizer, which processes the signal by reading the filters""coefficient values of each corresponding band and the remained status values of the nodes from the memory. And, it also includes the frequency domain equalizer, which processes the signal by reading the filters""coefficient values of each corresponding band.